1. Field of the Invention
The present invention relates to an x-ray image storage screen, and in particular to such a storage screen for use in exposure devices for converting x-ray images into video pictures.
2. Description of the Prior Art
A device for storing and displaying a radiation image wherein an x-ray image is converted into a video picture is described in U.S. Pat. No. 3,975,637. Devices of this type make use of phosphorescent or luminescent substances forming a storage layer, and the stored signals are obtained from the storage layer by the use of applied heat, such as a laser beam. In general, the incident x-ray image excites electrons in the storage material, these electrons being captured and retained. For readout, the layer is scanned line-by-line with an infrared beam, such as a laser beam. The captured electrons are then released and emit light. The emitted light is converted into a signal sequence corresponding to the scanning, such conversion occurring, for example, in an optical detector such as a secondary electron multiplier. This signal sequence can then be edited and displayed in a manner standard for television.
Highly sensitive luminescent systems which can be stimulated by radiation are required for such storage and readout. Known substances of this type, for example those described in the aforementioned U.S. Pat. No. 3,975,637, such as alkali and alkaline earth halogens, sulfates, borates, sulfides and silicates, however, are stimualted by infrared radiation at a level which is too low for use in medical radiology.
One attempted solution to this problem has been the use of activated barium fluorohalogen luminescent systems. A barium fluorohalogen system is described in European patent application 0 021 174, this system additionally containing a further alkaline earth or zinc or cadmium, and a europium activator. This system, however, exhibits too high an enertia for many uses, that is, the scan rate is too low.
Another barium fluorohalogen system which contains activators is described in European patent No. 0021342. One such system described therein consists of equal amounts of barium fluoride (BaF.sub.2) and barium bromide (BaBr.sub.2) and contains 0.1 mol strontium fluoride (SrF.sub.2) as well as europium chloride (EuCl.sub.3) as an activator. The stated purpose of this system is to achieve improved illumination intensity. This system, however is not chemically stable.